Method of manufacturing diamond substrate, diamond substrate, and diamond composite substrate

ABSTRACT

A method of manufacturing a diamond substrate includes: forming an ion implantation layer at a side of a main surface of a diamond seed substrate by implanting ions into the main surface of the diamond seed substrate; producing a diamond structure by growing a diamond growth layer by a vapor phase synthesis method on the main surface of the diamond seed substrate, after implanting the ions; and performing heat treatment on the diamond structure. The performed heat treatment causes the diamond structure to be separated along the ion implantation layer into a first structure including the diamond seed substrate and failing to include the diamond growth layer, and a diamond substrate including the diamond growth layer. Thus, the method of manufacturing a diamond substrate is provided that enables a diamond substrate with a large area to be manufactured in a short time and at a low cost.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a diamondsubstrate, and relates to a diamond substrate and a diamond compositesubstrate including this diamond substrate.

BACKGROUND ART

Diamond has many outstanding properties such as high hardness and highthermal conductivity, and additionally high light transmission rate andwide bandgap energy. Diamond is therefore used widely as a material forvarious tools, optics, semiconductor devices or electronic components,and will become still more important in the future.

In industry, in addition to natural diamonds, artificially synthesizeddiamonds having a stable quality are mainly used. Currently, most ofartificially synthesized single crystals of diamond are produced bysynthesis in a high-temperature high-pressure environment (diamond isstable in this environment) at a temperature on the order of onethousand and several hundred degrees to two thousand and several hundreddegrees and a pressure of several tens of thousands of atmospheres ormore (high-temperature high-pressure method).

An ultrahigh pressure vessel in which the aforementionedhigh-temperature high-pressure environment is to be generated is veryexpensive and restricted in size. Therefore, when a single crystaldiamond substrate is synthesized by the high-temperature high-pressuremethod, the size of the resultant single crystal diamond substrate islimited. With regard to Type Ib diamond containing nitrogen (N) as animpurity and having a yellowish color, a single crystal substrate ofType Ib diamond having a diameter of 10 mm is being manufactured by thehigh-temperature high-pressure method. However, the size on the order of10 mm in diameter is considered as a substantial limit. With regard totransparent and colorless Type IIa diamond containing no impurity, asingle crystal substrate of Type IIa diamond manufactured by thehigh-temperature high-pressure method has a diameter on the order ofonly several millimeters or less, except for natural Type IIa diamond.

In addition to the high-temperature high-pressure method, the vaporphase synthesis method is an established diamond synthesis method. Thevapor phase synthesis method can be used to grow a diamond crystalsubstrate with a relatively large area having a diameter on the order ofsix inches (152.4 mm). Usually, a polycrystal of diamond is obtained bythis method. When the diamond is used particularly as a material for anultraprecision tool or optic required to have a smooth surface, or as amaterial for a semiconductor device required to have a preciselycontrolled impurity concentration or high carrier mobility, for example,among industrial uses of the diamond, a single crystal of diamond isused. Therefore, how to epitaxially grow a single crystal diamondsubstrate by the vapor phase synthesis method is being studied.

Generally, epitaxial growth includes homoepitaxial growth producing agrowth layer of the same material as the material for the seedsubstrate, and heteroepitaxial growth producing a growth layer of amaterial different from the material for the seed substrate. It has beenconsidered difficult to grow a single crystal of diamond byheteroepitaxial growth. In recent years, a free-standing diamond filmhaving a diameter on the order of one inch (25.4 mm) has been formed,and thus growth of a single crystal diamond by heteroepitaxial growthhas remarkably advanced. However, the crystal quality of the singlecrystal of diamond obtained by the heteroepitaxial growth is inferior tothe crystal quality of a single crystal of diamond obtained by thehomoepitaxial growth. It is therefore considered preferable to study howto synthesize a single crystal diamond substrate by homoepitaxial growthto thereby establish a method of manufacturing a single crystal diamondsubstrate having a large area.

In the case of homoepitaxial growth, high-purity diamond is grown byvapor deposition on a Type Ib diamond substrate (seed substrate) whichis obtained by the high-temperature high-pressure method, for example,and thereafter the seed substrate is removed. In this way, a Type IIadiamond substrate having a larger area than a Type IIa diamond substrateobtained by the high-temperature high-pressure method can be produced.As disclosed in Japanese Patent Laying-Open No. 3-75298 (PTD 1), it hasbeen reported that a plurality of diamond substrates or diamond crystalsin the same crystal orientation can be used to integrally grow diamondthereon to obtain diamond having low angle grain boundaries only.

Problems in synthesis of a single crystal diamond substrate byhomoepitaxial growth are how to remove and how to reuse the seedsubstrate. When a Type Ib diamond substrate or the like is used as aseed substrate to manufacture a single crystal diamond substrate, it isnecessary to remove the seed substrate from the growth layer (which isto serve as a single crystal diamond substrate) by a certain method.This method may for example be a method to detach the growth layer fromthe seed substrate, or a method to completely remove the seed substrate.The seed substrate is formed of a single crystal of diamond andtherefore expensive. In view of this, it is preferable to employ theformer method. For example, a typical method is slicing with a laserbeam.

However, in the case of slicing with a laser beam, a growth layer havinga larger area requires a correspondingly greater thickness of the seedsubstrate or is accompanied by a decreased rate of success. Therefore,for a growth layer formed of a single crystal of diamond of 10 mm×10 mmfor example, it is difficult to detach the growth layer from the seedsubstrate by slicing with a laser beam and thus the method to completelyremove the seed substrate (the latter method) must be used. The methodto completely remove the seed substrate may polish the seed substratewith diamond abrasive grains, may cause the seed substrate to react withan iron surface so that a part of the seed substrate which has reactedwith the iron surface is removed, or may apply an ion beam to the seedsubstrate, for example. However, any method requires a long time tocompletely remove the seed substrate. Moreover, in the case of thismethod, the substrate (seed substrate) obtained by the high-temperaturehigh-pressure method cannot be reused and thus the cost formanufacturing the single crystal diamond substrate cannot be reduced.

Under the above circumstances, Japanese Patent Laying-Open No.2011-195407 (PTD 2) and Japanese Patent Laying-Open No. 2012-86988 (PTD3) each disclose a method of detaching a diamond layer (a part to serveas a single crystal diamond substrate) by implanting ions into thediamond seed substrate to form an electrically conductive non-diamondlayer and electrochemically etching away the non-diamond layer.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 3-75298-   PTD 2: Japanese Patent Laying-Open No. 2011-195407-   PTD 3: Japanese Patent Laying-Open No. 2012-86988

SUMMARY OF INVENTION Technical Problem

According to the technique disclosed in Japanese Patent Laying-Open Nos.2011-195407 and 2012-86988, the non-diamond layer is brought intocontact with an etching solution to be etched away. Due to the thinthickness of the non-diamond layer, the etch rate is considerably slow.Therefore, when the technique disclosed in Japanese Patent Laying-OpenNos. 2011-195407 and 2012-86988 is used to manufacture a single crystaldiamond substrate having a large area, the etching time for thenon-diamond layer is exceedingly long. Even when a single crystaldiamond substrate of 4 mm×4 mm is to be manufactured, etching of thenon-diamond layer takes 10 hours or more. It is therefore difficult tomanufacture a large-area single crystal diamond substrate in a shorttime and at a low cost using the technique disclosed in Japanese PatentLaying-Open Nos. 2011-195407 and 2012-86988.

According to Japanese Patent Laying-Open No. 2012-86988, improvements ofthe etching conditions are attempted. However, even when the etchingconditions are improved, the etching time for the non-diamond layer islonger when the area of the used diamond seed substrate is larger. Thus,when the etching conditions for the non-diamond layer are only improved,it is still difficult to manufacture a large-area single crystal diamondsubstrate in a short time and at a low cost. Even when ion implantationconditions are improved to increase the thickness of the non-diamondlayer, it is still difficult to manufacture a large-area single crystaldiamond substrate in a short time and at a low cost. Moreover, alarge-area polycrystal diamond has similar problems as well.

In view of these circumstances, it is an object to provide a method ofmanufacturing a diamond substrate that enables manufacture of alarge-area single crystal or polycrystal diamond substrate in a shorttime and at a low cost, provide a diamond substrate obtained by such amanufacturing method, and provide a diamond composite substrateincluding such a diamond substrate.

Solution to Problem

A method of manufacturing a diamond substrate according to an aspect ofthe present invention includes: forming an ion implantation layer at aside of a main surface of a diamond seed substrate by implanting ionsinto the main surface of the diamond seed substrate; producing a diamondstructure by growing a diamond growth layer by a vapor phase synthesismethod on the main surface of the diamond seed substrate, afterimplanting ions; and performing heat treatment on the diamond structure.The performed heat treatment causes the diamond structure to beseparated along the ion implantation layer into: a first structureincluding the diamond seed substrate and failing to include the diamondgrowth layer; and a diamond substrate including the diamond growthlayer.

Advantageous Effects of Invention

Based on the foregoing, a diamond substrate having a large area can bemanufactured in a short time and at a low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows cross-sectional views (A), (B), and (C) illustrating amethod of manufacturing a diamond substrate in the present embodiment inorder of steps.

FIG. 2 shows a graph for results of measurement, by secondary ion massspectrometry, of a concentration distribution of first atoms in thevicinity of an ion implantation layer before separation of a diamondsubstrate.

FIG. 3 shows a cross-sectional view of a diamond substrate in thepresent embodiment and a schematic diagram of a concentrationdistribution of first atoms in a relevant part of the diamond substrate.

FIG. 4 is a schematic diagram of a photoluminescence spectrum of adiamond substrate in the present embodiment.

FIG. 5 is a schematic diagram of an absorption spectrum of a diamondsubstrate in the present embodiment.

FIG. 6 is a cross-sectional view of a diamond composite substrate in thepresent embodiment.

DESCRIPTION OF EMBODIMENTS

As described above, it is difficult for the technique disclosed inJapanese Patent Laying-Open Nos. 2011-195407 and 2012-86988 tomanufacture a large-area single crystal diamond substrate (a singlecrystal diamond substrate having a diameter of 50.8 mm or more (twoinches or more) for example) in a short time and at a low cost. Theinventors of the present invention thus considered it is necessary tofind a method of manufacturing a single crystal diamond substratecompletely different from the method disclosed in Japanese PatentLaying-Open Nos. 2011-195407 and 2012-86988, in order to manufacture alarge-area single crystal diamond substrate in a short time and at a lowcost.

Japanese Patent Laying-Open Nos. 2006-210660 and 2011-61084 for exampledisclose a method of manufacturing a laminated substrate produced bylaminating a nitride semiconductor film and a silicon substrate into thelaminated substrate. Specifically, ions are implanted in the vicinity ofa surface of the nitride semiconductor substrate. The silicon substrateis superposed on the surface of the nitride semiconductor substrate fromwhich the ions are implanted, and thereafter heat treatment isperformed. This heat treatment causes a part of the nitridesemiconductor substrate to be parted together with the silicon substratefrom the remaining bulk part of the nitride semiconductor substrate,along the layer formed of implanted ions (ion implantation layer). Inthis way, the aforementioned laminated substrate is obtained. Theinventors of the present invention considered that the above-describedmethod of manufacturing a laminated substrate can be used to manufacturea large-area single crystal diamond substrate in a short time and at alow cost. However, it had been considered difficult to manufacture asingle crystal diamond substrate through heat treatment. In order toconfirm this common technical knowledge, the inventors of the presentinvention implanted ions into a diamond seed substrate, grew a diamondlayer on the surface of the diamond seed layer from which the ions wereimplanted, and thereafter performed heat treatment. Consequently, fromsome diamond seed substrates, a single crystal diamond substrate wasdetached along the ion implantation layer (such detachment of thediamond substrate is hereinafter referred to as “separation of diamondsubstrate”). However, in some diamond seed substrates, separation of asingle crystal diamond substrate was not confirmed.

A reason for occurrence of the separation of the diamond substrate maybe considered as follows. The heat treatment performed on the diamondseed substrate in which ions are implanted causes the ions to gather ina planar form in the ion implantation layer and form air bubbles.Accordingly, fine voids are formed to spread two-dimensionally in theion implantation layer, resulting in separation of the diamondsubstrate.

Meanwhile, the inventors of the present invention examined in detail thediamond seed substrates where separation of the diamond substrate didnot occur, and considered that separation of the diamond substrate didnot occur for the following reasons.

A first reason is considered as transformation of diamond to graphite oramorphous carbon due to the ion implantation. When ions are implantedinto the diamond seed substrate, diamond in the region where the ionsare implanted may be transformed to graphite or amorphous carbon. In theregion where diamond is transformed to graphite or amorphous carbon, atleast a part of the ions implanted into the diamond seed substrate isfixed. When heat treatment is performed on such a diamond seedsubstrate, the implanted ions are difficult to diffuse in the ionimplantation layer and therefore difficult to gather in a planar form.Accordingly, fine voids are difficult to form and spreadtwo-dimensionally in the ion implantation layer. Because of this,separation of the diamond substrate is less likely to occur.

A second reason is considered as the strength of cohesion between carbonatoms forming a diamond crystal structure. The cohesion between carbonatoms forming the diamond crystal structure is known to be significantlystrong. Therefore, depending on heat treatment conditions, bonds betweencarbon atoms forming the diamond crystal structure are difficult tosever and thus separation of the diamond substrate is less likely tooccur. Moreover, even when the heat treatment conditions are changed sothat the bonds between carbon atoms forming the diamond crystalstructure are easily severed, the heat treatment may causetransformation of diamond to graphite or amorphous carbon. As a resultof this, the crystal quality of the diamond substrate is deteriorated.

The inventors of the present invention further conducted studiesintensively in view of the foregoing, and devised a method enabling alarge-area single crystal or polycrystal diamond substrate to bemanufactured in a short time and at a low cost.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

First of all, aspects of the present invention will be described one byone.

[1]A method of manufacturing a diamond substrate according to an aspectof the present invention includes: forming an ion implantation layer ata side of a main surface of a diamond seed substrate by implanting ionsinto the main surface of the diamond seed substrate; producing a diamondstructure by growing a diamond growth layer by a vapor phase synthesismethod on the main surface of the diamond seed substrate, afterimplanting the ions; and performing heat treatment on the diamondstructure. The performed heat treatment causes the diamond structure tobe separated along the ion implantation layer into a first structureincluding the diamond seed substrate and failing to include the diamondgrowth layer, and a diamond substrate including the diamond growthlayer. Thus, a diamond substrate with a large area can be manufacturedin a short time and at a low cost.

“Diamond seed substrate” refers to a substrate which is used formanufacturing a diamond substrate and on which a diamond growth layer isto be grown.

“Diamond structure” includes the diamond seed substrate and the diamondgrowth layer formed on the main surface of the diamond seed substrate.

[2] Preferably, the ions include ions of one kind of atom selected fromthe group consisting of hydrogen atom, helium atom, nitrogen atom,oxygen atom, and argon atom. Thus, easy separation of the diamondsubstrate having an excellent crystal quality can be implemented.

[3] Preferably, the ions are implanted with an ion implantation energyof not less than 10 keV and not more than 500 keV to an ion implantationdepth of not more than 3 μm. Thus, the diamond substrate can beseparated easily.

[4] Preferably, the ions are implanted at a dose of not less than 1×10¹⁶cm⁻² and not more than 1×10¹⁸ cm⁻². Thus, the diamond substrate can beseparated easily.

[5] Preferably, the heat treatment is performed in one of an inert gasatmosphere containing oxygen gas at a concentration of not less than 10ppm and not more than 1000 ppm, and a vacuum atmosphere at a vacuum ofnot less than 1×10⁻⁸ Pa and not more than 1×10⁻² Pa containing oxygengas at a partial pressure of not less than 1×10⁻⁸ Pa and not more than1×10⁻⁵ Pa. Thus, the diamond substrate can be separated smoothly.

[6] Preferably, a nitrogen concentration at the main surface of thediamond seed substrate is not more than 100 ppm. Thus, the diamondsubstrate can be separated reliably.

[7] Preferably, the diamond seed substrate is grown by a vapor phasesynthesis method. Thus, the diamond substrate can be separated reliably.

[8] The diamond seed substrate excluding the ion implantation layer mayinclude a layer having a resistivity of not less than 10⁻⁵ Ω·cm and notmore than 10⁹ Ω·cm. Alternatively, the diamond seed substrate may have aresistivity of not less than 10⁻⁵ Ω·cm and not more than 10⁹ Ω·cm. Evenin such a case, the diamond substrate can be separated.

[9] The diamond seed substrate may be a single crystal and the diamondgrowth layer may be a single crystal. Even in such a case, the diamondsubstrate can be separated.

[10] The diamond seed substrate may be a polycrystal and the diamondgrowth layer may be a polycrystal. Even in such a case, the diamondsubstrate can be separated.

[11] Preferably, crystal grains in the polycrystal at the main surfaceof the diamond seed substrate have an average grain size of not lessthan 30 μm. Thus, easy separation of the polycrystal diamond seedsubstrate can be implemented.

[12] A diamond substrate according to an aspect of the present inventionis manufactured in accordance with the method of manufacturing a diamondsubstrate according to an aspect of the present invention. The diamondsubstrate 50 manufactured in accordance with this method has a highcrystal quality.

[13] A diamond substrate according to another aspect of the presentinvention is a diamond substrate of a single crystal. Aphotoluminescence spectrum of the diamond substrate includes: a firstemission peak having an emission peak wavelength in a wavelength rangeof not less than 450 nm and not more than 650 nm; and a second emissionpeak having an emission peak wavelength in a wavelength range of notless than 570 nm and not more than 580 nm. The photoluminescencespectrum is obtained by applying, to the diamond substrate, excitationlight having a peak wavelength in a wavelength range of not less than315 nm and not more than 335 nm at a temperature in a temperature rangeof not less than 7 K and not more than 83 K. The first emission peak hasa full width at half maximum of not less than 50 nm. The second emissionpeak has a full width at half maximum of not more than 10 nm. A peakheight of the first emission peak is not less than 0.1 times ( 1/10times) as high as a peak height of the second emission peak.

[14] A diamond substrate according to still another aspect of thepresent invention is a diamond substrate of a single crystal. Anabsorption spectrum of the diamond substrate with wavelength plotted ona horizontal axis and absorption coefficient plotted on a vertical axisincludes a first absorption peak having an absorption peak wavelength ina wavelength range of not less than 265 nm and not more than 275 nm, anda second absorption peak having an absorption peak wavelength in awavelength range of not less than 370 nm and not more than 390 nm. Apeak height of the second absorption peak is lower than a peak height ofthe first absorption peak.

[15] A diamond substrate according to a further aspect of the presentinvention is a diamond substrate of a polycrystal. The diamond substrateis obtained by a vapor phase synthesis method, and crystal grains in thepolycrystal of the diamond substrate have an average grain size of notless than 30 μm.

[16] The diamond substrate may include a first surface and a secondsurface located opposite to the first surface. Concentration of firstatoms at the second surface may be higher than a concentration of thefirst atoms at the first surface, the first atoms being different fromatoms forming a diamond crystal structure. The concentration of thefirst atoms may decrease from the second surface toward an inside of thediamond substrate, in a thickness direction of the diamond substrate.The concentration of the first atoms at the second surface may be notless than 100 times as high as the concentration of the first atoms inthe inside of the diamond substrate.

“First atoms different from atoms forming a diamond crystal structure”include not only atoms other than carbon atoms but also carbon atomswhich do not form the diamond crystal structure. “First atoms at thesecond surface” do not include first atoms naturally adhering to thesecond surface.

[17] Preferably, in at least a part of the second surface, a meshstructure defined by a protrusion or depression with a height of notless than 1 nm and not more than 50 nm surrounding flat regions isformed. Preferably, the flat regions of the second surface each have asize of not less than 3 μm and not more than 30 μm. Thus, the diamondsubstrate can be used as a substrate to be mounted with a device.

“A protrusion or depression with a height of not less than 1 nm and notmore than 50 nm” includes a protrusion with a height of not less than 1nm and not more than 50 nm and a depression with a depth of not lessthan 1 nm and not more than 50 nm.

“Flat region” refers to a region where no protrusion with a height of 1nm or more and no depression with a depth of 1 nm or more is formed. Inother words, “flat region” includes not only a region where theprotrusion/depression is not formed at all but also a region where atleast one of a protrusion with a height of less than 1 nm and adepression with a depth of less than 1 nm is formed.

The “size of each of the flat regions of the second surface” refers tothe diameter when the flat region of the second surface is circular inshape, or refers to a maximum dimension when the flat region has a shapedifferent from the circular shape.

[18] Preferably, the second surface is a physically polished surface.Thus, the second surface is made planer sufficiently. “Physicallypolished” means that it is polished by a method different from polishingwith a chemical (chemical polishing), and means that the surface ispolished through cutting, wear, or deformation.

[19] Preferably, the diamond substrate has a thickness of not less than100 μm and not more than 1500 μm. Thus, the diamond substrate can beused as a substrate to be mounted with a device.

[20] Preferably, the diamond substrate has a diameter of not less than50.8 mm (two inches). Thus, the diamond substrate can be used as asubstrate to be mounted with a device.

[21] The diamond substrate may include a layer having a resistivity ofnot less than 10⁻⁵ Ω·cm and not more than 10⁹ Ω·cm. Alternatively, thediamond substrate may be a substrate having a resistivity of not lessthan 10⁻⁵ Ω·cm and not more than 10⁹ Ω·cm. Such a diamond substrate canbe obtained by manufacturing the diamond substrate in accordance withthe method of manufacturing a diamond substrate in an aspect of thepresent invention.

[22] A diamond composite substrate according to an aspect of the presentinvention includes: a diamond substrate according to an aspect of thepresent invention; and an epitaxial layer formed through epitaxialgrowth on at least one of a first surface of the diamond substrate and asecond surface of the diamond substrate located opposite to the firstsurface.

DETAILS OF EMBODIMENTS OF THE INVENTION

Hereinafter, an embodiment of the present invention (hereinafterreferred to as “present embodiment”) will be further detailed. In thedrawings, the same reference characters denote the same or correspondingparts. The relation between dimensions such as length, width, thickness,and depth is changed as appropriate for the sake of clarification andsimplification of the drawings, and does not represent the actualdimensional relation.

[Method of Manufacturing Diamond Substrate]

FIG. 1 shows cross-sectional views (A), (B), and (C) illustrating amethod of manufacturing a diamond substrate 50 in the present embodimentin order of steps.

The method of manufacturing a diamond substrate 50 in the presentembodiment includes the steps of: forming an ion implantation layer 15at a side of a main surface 11 of a diamond seed substrate 10 byimplanting ions into main surface 11 of diamond seed substrate 10(hereinafter also referred to as ion implantation step); producing adiamond structure 30 by growing a diamond growth layer 20 by a vaporphase synthesis method on main surface 11 of diamond seed substrate 10,after implanting the ions (hereinafter also referred to asdiamond-growth-layer growth step); and performing heat treatment ondiamond structure 30 (hereinafter also referred to as heat treatmentstep). In this way, the diamond substrate having a large area can bemanufactured in a short time and at a low cost. The method ofmanufacturing diamond substrate 50 in the present embodiment isappropriately applied regardless of whether the diamond substrate is asingle crystal or a polycrystal, except that the method is speciallydefined herein below.

A method of manufacturing a diamond substrate other than theabove-described method in the present embodiment may include the stepsof implanting ions into the main surface of the diamond seed substrate,forming an intermediate layer on the main surface of the diamond seedsubstrate after implanting ions, producing a diamond structure includingthe intermediate layer by growing a diamond growth layer on a mainsurface of the intermediate layer, and performing heat treatment on thediamond structure including the intermediate layer. However, in the caseof such a manufacturing method producing the diamond structure includingthe intermediate layer, the diamond growth layer is epitaxially grown onthe main surface of the intermediate layer of any material other thandiamond (heteroepitaxial growth), and thus it is difficult to obtain thediamond growth layer having a high crystal quality. In contrast, in thecase of the method of manufacturing a diamond substrate in the presentembodiment, the diamond growth layer is epitaxially grown on the mainsurface of the diamond seed substrate without the intermediate layerinterposed therebetween (homoepitaxial growth) after implanting ions,and thus the diamond growth layer having a high crystal quality isobtained.

If the seed substrate is any substrate other than diamond seedsubstrate, such as silicon seed substrate, germanium seed substrate, orgallium nitride seed substrate, the main surface in which ions areimplanted has a significant damage to the crystal. Therefore, it isdifficult to homoepitaxially grow a silicon growth layer, a germaniumgrowth layer, and gallium nitride growth layer on respective mainsurfaces of the corresponding substrates. However, in the diamond seedsubstrate, the main surface in which ions are implanted underappropriate ion implantation conditions is hardly damaged and therefore,a diamond growth layer can be deposited on the main surface byhomoepitaxial growth.

A reason for this is as follows. In diamond seed substrate 10, thestrength of cohesive between carbon atoms forming the diamond crystalstructure is considerably large. Therefore, even when ions areimplanted, bonds between carbon atoms are difficult to sever and thusthe main surface is hardly damaged. Therefore, separation of diamondseed substrate 10 requires a greater energy (higher temperature) ascompared with the silicon seed substrate, the germanium seed substrate,and the gallium nitride seed substrate. It is therefore important tocontrol ion implantation conditions, conditions for growing the diamondgrowth layer, and heat treatment conditions so that diamond seedsubstrate 10 will not be separated at the growth temperature or lowerfor growing diamond growth layer 20 after ion implantation, but diamondseed substrate 10 is separated along ion implantation layer 15 at thetemperature of the subsequent heat treatment.

<Ion Implantation Step>

In the ion implantation step, ions are implanted into main surface 11 ofdiamond seed substrate 10. Accordingly, ion implantation layer 15 isformed at a side of main surface 11 of diamond seed substrate 10 (FIG. 1(A)).

Diamond Seed Substrate

Diamond seed substrate 10 may be either a single crystal or apolycrystal. When a single crystal diamond substrate is to bemanufactured, it is preferable to use a single crystal diamond seedsubstrate. When a polycrystal diamond substrate is to be manufactured,it is preferable to use a polycrystal diamond seed substrate. Singlecrystal diamond seed substrate 10 may for example be Type Ib diamondsubstrate, Type IIa diamond substrate, or a diamond substrate formed bythe vapor phase synthesis method (CVD (Chemical Vapor Deposition) methodfor example). Polycrystal diamond seed substrate 10 may be a diamondsubstrate formed by the vapor phase synthesis method (CVD method forexample).

When polycrystal diamond substrate 50 is to be manufactured by growingpolycrystal diamond growth layer 20 by the vapor phase synthesis methodon main surface 11 of polycrystal diamond seed substrate 10, the averagegrain size of crystal grains in the polycrystal of main surface 11 ofdiamond seed substrate 10 is preferably 30 μm or more, more preferably60 μm or more, and still more preferably 90 μm or more, in order to makeit easy to separate diamond substrate 50 including the grown diamondgrowth layer 20. In ion implantation layer 15 formed at a side of mainsurface 11 of polycrystal diamond seed substrate 10, the heat treatmentas will be described later herein causes ions to be gasified to generateair bubbles. However, at grain boundaries, air bubble growth is stoppeddue to trapping of ions and/or gas leakage. Therefore, if the averagegrain size of crystal grains in the polycrystal of main surface 11 ofdiamond seed substrate 10 is larger than 30 μm, the air bubble diameteris large enough, the air bubble density is high enough, and the gasdissociation pressure is large enough to make it easy to separatediamond substrate 50. The average grain size of crystal grains in thepolycrystal of main surface 11 of diamond seed substrate 10 is observedwith an SEM (Scanning Electron Microscope).

“Average grain size of crystal grains” herein refers to the averagegrain size of crystal grains appearing on the main surface. As to how tocalculate the average grain size, the number of crystal grains within ameasurement range is counted, the total area of the measurement range isdivided by the number of crystal grains to determine the area percrystal grain, and the radius is calculated supposing that the crystalgrains are circular in shape to define the average crystal grain size asthe determined value of the radius.

The nitrogen concentration of main surface 11 of diamond seed substrate10 is preferably 100 ppm or less, more preferably 80 ppm or less, stillmore preferably 10 ppm or less, further preferably 5 ppm or less, andparticularly preferably 0.1 ppm or less. Accordingly, it is possible toeffectively prevent transformation of diamond seed substrate 10 tographite or amorphous carbon due to ion implantation. Thus, through heattreatment as described later herein, diamond structure 30 (FIG. 1 (B))is easily divided along ion implantation layer 15 into a first structure40 which includes diamond seed substrate 10 and does not include diamondgrowth layer 20, and diamond substrate 50 which includes diamond growthlayer 20 (FIG. 1 (C)). The division of diamond structure 30 into firststructure 40 and diamond substrate 50 along ion implantation layer 15 ishereinafter referred to as “separation of diamond substrate 50.”

If transformation of diamond seed substrate 10 to graphite or amorphouscarbon due to ion implantation can be prevented effectively, diamondsubstrate 50 can be separated without raising the heat treatmenttemperate to an extremely high temperature for the heat treatment aswill be described later herein. Thus, transformation of diamond growthlayer 20 to graphite or amorphous carbon due to the heat treatment canbe prevented. Moreover, if it is unnecessary to raise the heat treatmenttemperature to an extremely high temperature for the heat treatment,separation of diamond substrate 50 occurs reliably. Accordingly, diamondsubstrate 50 can be manufactured without causing generation of cracks.For the above-described reasons, diamond substrate 50 excellent incrystal quality can be manufactured.

It is practically preferable to use diamond seed substrate 10 having anitrogen concentration of 5 ppm or less at main surface 11. In thiscase, diamond substrate 50 can be separated more easily by the heattreatment as will be described later herein (FIG. 1 (C)), and diamondsubstrate 50 more excellent in crystal quality can be manufactured.Diamond seed substrate 10 having a nitrogen concentration of less than 1ppm is difficult to obtain. However, as long as the nitrogenconcentration at main surface 11 is not less than 1 ppm and not morethan 5 μm, diamond substrate 50 can be separated still more easily bythe heat treatment. Thus, diamond seed substrate 10 having a nitrogenconcentration at main surface 11 of not less than 1 ppm and not morethan 5 ppm is appropriately used.

In the Type Ib diamond substrate mentioned above as an example of singlecrystal diamond seed substrate 10, there is a large variation in thenitrogen concentration in main surface 11. Usually, this nitrogenconcentration is 3 ppm to 100 ppm, preferably on the order of 3 ppm to80 ppm, while some such diamond seed substrates have a maximum nitrogenconcentration higher than 100 ppm. It is preferable to use a Type Ibdiamond substrate having a nitrogen concentration of 100 ppm or less atmain surface 11, in order to prevent generation of cracks. The Type Ibdiamond substrate as an example of single crystal diamond seed substrate10 has a nitrogen concentration at main surface 11 of not more than 10ppm, preferably not more than 0.1 ppm. Moreover, a structure(hereinafter referred to as “CVD diamond substrate”) obtained by growinga diamond layer by the CVD (Chemical Vapor Deposition) method, which isone of vapor phase synthesis methods, on the upper surface of a Type Ibdiamond substrate instead of Type IIa diamond substrate may be used. TheCVD diamond substrate can be used as diamond seed substrate 10 toprovide diamond seed substrate 10 at a low price, and thus effectivelyreduce the cost for manufacturing diamond substrate 50. When the CVDdiamond substrate is used as diamond seed substrate 10, the nitrogenconcentration at the main surface of the diamond layer grown by the CVDmethod is 5 ppm or less. Therefore, it is preferable to implant ionsinto the diamond layer grown by the CVD method.

In a CVD diamond substrate, which is mentioned as an example ofpolycrystal diamond seed substrate 10, obtained by the CVD method as akind of vapor phase synthesis method, the nitrogen concentration at themain surface of the diamond layer grown by the CVD method is 5 ppm orless. It is thus preferable to implant ions into the diamond layer grownby the CVD method.

Accordingly, the nitrogen concentration at main surface 11 of diamondseed substrate 10 is preferably 100 ppm or less, more preferably 80 ppmor less, still more preferably 10 ppm or less, particularly preferably 5ppm or less, and most preferably 0.1 ppm or less.

Diamond seed substrate 10 may have a uniform nitrogen concentration ornon-uniform nitrogen concentration. A region in diamond seed substrate10 with no implanted ions in the region (a region which is a part ofdiamond seed substrate 10 and located opposite to main surface 11, forexample) may have a nitrogen concentration of 100 ppm or less or morethan 100 ppm. The nitrogen concentration at main surface 11 of diamondseed substrate 10 is herein measured by SIMS (secondary ion massspectrometry).

Preferably, diamond seed substrate 10 is grown by a vapor phasesynthesis method. The vapor phase synthesis method is not particularlylimited. However, in order to reduce the nitrogen concentration at mainsurface 11, CVD (Chemical Vapor Deposition) method, more specificallymicrowave plasma CVD method, hot filament CVD method, or the like ispreferred. Such a vapor phase synthesis method can be used to preventgeneration of cracks and separate diamond reliably.

The resistivity of diamond seed substrate 10 is not particularlylimited. According to the method disclosed for example in JapanesePatent Laying-Open No. 2011-195407, the non-diamond layer is anelectrically conductive layer and is electrochemically etched(electrolytic etching) in a subsequent step. Therefore, if anelectrically conductive substrate is used as a substrate into which ionsare implanted, the electrolytic etching for the ion implantation layeris not effectively performed. Thus, the electrically conductivesubstrate cannot be used as a substrate in which ions are implanted.

Regarding the method in the present embodiment to perform heat treatmentafter ion implantation, the heat treatment described later herein causesimplanted ions to gather in a planar form in the ion implantation layer15 and form air bubbles. Accordingly, separation of diamond substrate 50occurs. Thus, regardless of whether diamond seed substrate 10 is anelectrically conductive substrate or insulating substrate, diamondsubstrate 50 can be separated. For example, even when diamond seedsubstrate 10 excluding ion implantation layer 15 includes a layer havinga resistivity of not less than 10⁻⁵ Ω·cm and not more than 10⁻² Ω·cm toexhibit electrical conductivity, a resistivity of not less than 10⁶ Ω·cmand not more than 10⁹ Ω·cm to exhibit electrical insulation, or aresistivity between them, or even when the whole diamond seed substrate10 including ion implantation layer 15 has a resistivity of not lessthan 10⁻⁵ Ω·cm and not more than 10⁻² Ω·cm to exhibit electricalconductivity, a resistivity of not less than 10⁶ Ω·cm and not more than10⁹ Ω·cm to exhibit electrical insulation, or a resistivity betweenthem, diamond substrate 50 can be separated. As mentioned above, theresistivity of the diamond seed substrate is not particularly limited interms of both electrical insulation and electrical conductivity, theresistivity is set to not less than 10⁻⁵ Ω·cm and not more than 10⁹ Ω·cmas a range for practical production of a single crystal or polycrystaldiamond. The resistivity can be measured by the four-probe method or thefour-terminal method. When the resistivity is measured by thefour-terminal method, it is preferable to use an electrode formed of aTi/Pt/Au multilayer structure as an electrode.

While main surface 11 of diamond seed substrate 10 is preferably asurface having plane orientation (001), the main surface may be asurface having an off angle of 5 degrees or less to plane orientation(001).

The thickness of diamond seed substrate 10 is not particularly limited,and is preferably not less than 50 μm and not more than 1000 μm, andmore preferably not less than 200 μm and not more than 500 μm.

Ion Implantation Conditions

The growth temperature for diamond growth layer 20 is very high asdescribed above. Therefore, during growth of diamond growth layer 20,separation may start locally in diamond seed substrate 10 in which ionsare implanted, and cracks may be generated. In this case, it may beimpossible to neatly detach diamond substrate 50 from ion implantationlayer 15 after forming diamond substrate 50. In view of this, in orderto avoid the start of the separation at or less than the growthtemperature for diamond growth layer 20, it is preferable to define ionimplantation conditions under which diamond bonds are not severed, andto select, as atoms for ion implantation, atoms which have a smallatomic radius and thus do not sever carbon bonds or atoms which havehigh affinity with carbon and thus high tendency to combine with carbon.

While the ions to be implanted into diamond seed substrate 10 are notparticularly limited, it is preferable to use ions of first atoms whichare different from atoms forming a diamond crystal structure, and it ismore preferable that the ions include ions of one kind of atom selectedfrom the group consisting of hydrogen atom, helium atom, nitrogen atom,oxygen atom, and argon atom, which are to form gas in the form of atomsor molecules. In this way, transformation of diamond seed substrate 10to graphite or amorphous carbon due to ion implantation can be preventedeffectively. Accordingly, as described above, the heat treatmentdescribed later herein can be performed to easily separate diamondsubstrate 50 and to manufacture diamond substrate 50 which is excellentin crystal quality.

More preferably, the first atoms are atoms of a light element such ashydrogen or helium. Then, transformation of diamond seed substrate 10 tographite or amorphous carbon due to ion implantation can be preventedmore effectively. Accordingly, the heat treatment described later hereincan be performed to separate diamond substrate 50 more easily and tomanufacture diamond substrate 50 which is more excellent in crystalquality. The method to implant ions of the first atoms into diamond seedsubstrate 10 may be any method known as an ion implantation method,without being limited to a specific method.

The ion implantation energy is preferably not less than 10 keV and notmore than 500 keV. With an ion implantation energy of not less than 10keV, it is possible to prevent an excessively short ion implantationdepth (the distance between main surface 11 and a position in diamondseed substrate 10 at which the concentration of implanted ions ishighest, corresponding to the average projected range obtained bysimulation). Under this condition, it is possible to prevent damage tomain surface 11 of diamond seed substrate 10 due to implanted ions (suchas deterioration of the crystal quality of main surface 11 for example).On main surface 11 of diamond seed substrate 10, diamond growth layer 20is formed (FIG. 1 (B)). Therefore, as long as damage to main surface 11of diamond seed substrate 10 can be prevented, the crystal quality ofdiamond growth layer 20 can be enhanced and thus diamond substrate 50excellent in crystal quality can be manufactured.

With an ion implantation energy of not more than 500 keV, it is possibleto prevent an excessively large variation of ions (implanted ions) inthe thickness direction of diamond seed substrate 10 (this variation isrepresented by the standard deviation of the projected range,hereinafter referred to as “variation in the ion implantation depth”),and thus prevent an excessively low ion density in ion implantationlayer 15. Under this condition, the heat treatment described laterherein can be performed to cause the implanted ions to effectivelygather in a planar form in ion implantation layer 15. Moreover, with anion implantation energy of not more than 500 keV, transformation ofdiamond seed substrate 10 to graphite or amorphous carbon due to ionimplantation can be prevented effectively. Under these conditions, theheat treatment can be performed to easily separate diamond substrate 50.The ion implantation energy is more preferably not less than 50 keV andnot more than 200 keV.

The ion implantation dose is preferably not less than 1×10¹⁶ cm⁻² andnot more than 1×10¹⁸ cm⁻². With an ion implantation dose of not lessthan 1×10¹⁶ cm⁻², ions of an amount of necessary for separating diamondsubstrate 50 by the heat treatment described later herein are implantedinto diamond seed substrate 10. Then, the heat treatment can beperformed to easily separate diamond substrate 50.

With an ion implantation dose of not more than 1×10¹⁸ cm⁻²,transformation of diamond seed substrate 10 to graphite or amorphouscarbon due to ion implantation can effectively be prevented. Under thiscondition, as described above, the heat treatment described later hereincan be performed to easily separate diamond substrate 50, andmanufacture diamond substrate 50 which is excellent in crystal quality.The ion implantation dose is more preferably not less than 5×10¹⁶ cm⁻²and not more than 5×10¹⁷ cm⁻².

The ion implantation depth is preferably 3 μm or less. The variation inthe ion implantation depth is preferably 0.1 μm or less. Under theseconditions, diamond substrate 50 can be separated easily and reliably.In view of the above, it is preferable to set the ion implantationconditions in the above-described manner.

<Diamond-Growth-Layer Growth Step>

In the diamond-growth-layer growth step after the ion implantation step,diamond growth layer 20 is grown by the vapor phase synthesis method onmain surface 11 of diamond seed substrate 10. By the vapor phasedeposition method, diamond growth layer 20 of a single crystal isepitaxially grown on main surface 11 of diamond seed substrate 10 of asingle crystal, or diamond growth layer 20 of a polycrystal isepitaxially grown on main surface 11 of the diamond seed substrate of apolycrystal. In this way, diamond structure 30 of a single crystal or apolycrystal can be produced.

An example of the vapor phase synthesis method may be the CVD method. Asconditions for the vapor phase synthesis method, conditions known asconditions for forming a diamond layer by the CVD method can be usedwithout being limited to specific conditions. An example method may bethe following method. Methane and hydrogen are used as raw material gas,and the raw material gas is reacted in a plasma to deposit the reactant(diamond) on main surface 11 of diamond seed substrate 10 heated toapproximately 900 to 1100° C.

The resistivity of diamond structure 30 may be defined similarly to theresistivity of diamond seed substrate 10. In other words, diamondstructure 30 excluding ion implantation layer 15 may include a layerhaving a resistivity of not less than 10⁻⁵ Ω·cm and not more than 10⁻²Ω·cm to exhibit electrical conductivity, a resistivity of not less than10⁶ Ω·cm and not more than 10⁹ Ω·cm to exhibit electrical insulation, ora resistivity between them. Alternatively, the whole diamond structure30 including ion implantation layer 15 may have a resistivity of notless than 10⁻⁵ Ω·cm and not more than 10⁻² Ω·cm to exhibit electricalconductivity, a resistivity of not less than 10⁶ Ω·cm and not more than10⁹ Ω·cm to exhibit electrical insulation, or a resistivity betweenthem. In any case, the heat treatment described below can be performedto separate diamond substrate 50.

<Heat Treatment Step>

In the heat treatment step after the diamond-growth-layer growth step,the heat treatment is performed on diamond structure 30. This heattreatment causes the implanted ions to gather in a planar form in ionimplantation layer 15 and form air bubbles.

FIG. 2 shows the results of measurement, by SIMS, of the concentrationdistribution of first atoms (hydrogen atoms in this case) in thevicinity of ion implantation layer 15 before separation of diamondsubstrate 50 occurs. In FIG. 2 , on the left side of L20, theconcentration of first atoms at a side of ion implantation layer 15 inthe region which is to form a first structure 40 is shown and, on theright side of L20, the concentration of first atoms at a side of ionimplantation layer 15 in the region which is to form diamond substrate50 is shown. L21 represents the concentration distribution of firstatoms after the hydrogen ion implantation and before the heat treatment,and L22 represents the concentration distribution of first atoms afterthe heat treatment. SIMS is used to measure the concentrationdistribution of first atoms. It is considered that, in diamond structure30, ions of first atoms are distributed as indicated by theconcentration distribution shown in FIG. 2 . The concentrationdistributions of first atoms in diamond structure 30 (namely firststructure 40 and diamond substrate 50) before the heat treatment andafter the heat treatment as shown in FIG. 2 are substantially common todiamond structure 30 of a single crystal and diamond structure 30 of apolycrystal.

Before the heat treatment, the concentration distribution of first atomshad a peak including a vertex X (L21). However, after the heattreatment, the shape of the concentration distribution of first atomschanged to a shape with a section in the vicinity of vertex X cut off,and thus the maximum value of the concentration of first atoms decreased(L22). Based on the above, it is considered that the heat treatment isperformed to cause the implanted ions to gather in a planar form in ionimplantation layer 15 and form air bubbles (ions are eliminated).

When the implanted ions gather in a planar form in ion implantationlayer 15 to form air bubbles, fine voids spread two-dimensionally in ionimplantation layer 15, and accordingly diamond substrate 50 isseparated. Namely, ion implantation layer 15 is divided into a first ionimplantation layer 15 a and a second ion implantation layer 15 b andconsequently diamond structure 30 is divided into first structure 40 anddiamond substrate 50 (FIG. 1 (C)). In this way, diamond substrate 50 ismanufactured.

The heat treatment is considered as causing ions to gather and form airbubbles at a plurality of sites simultaneously in ion implantation layer15. Ion implantation layer 15 is formed in parallel with main surface 11of diamond seed substrate 10 (FIG. 1 (A)). Therefore, the separationtime of diamond substrate 50 (the time taken to separate diamondsubstrate 50) does not depend on the size of diamond substrate 50.

Namely, even a diamond substrate 50 having a large area (a singlecrystal diamond substrate having a diameter of 50.8 mm (two inches) ormore or a polycrystal diamond substrate having a diameter of 152.4 mm(six inches) or more, for example) is to be manufactured, diamondsubstrate 50 can be separated in a short time and thus can bemanufactured in a short time and at a low cost. The inventors of thepresent invention confirmed that even a single crystal diamond substratewith a small area (a single crystal diamond substrate of 4 mm×4 mm forexample) can be manufactured in a shortened time in accordance with themethod of manufacturing diamond substrate 50 in the present embodiment,relative to a single crystal diamond substrate with a small areamanufactured in accordance with the method disclosed for example inJapanese Patent Laying-Open No. 2011-195407 (Example described laterherein).

The separated surface of first structure 40 (upper surface in FIG. 1(C)) can be chemically or physically polished to reuse first structure40 as diamond seed substrate 10. Moreover, since the ion implantationdepth is preferably 3 μm or less, the part of diamond seed substrate 10to be lost when diamond substrate 50 is manufactured can be made small.Under these conditions, the number of diamond substrates 50 manufacturedfrom one diamond seed substrate 10 can be increased and accordingly thecost for manufacturing diamond substrate 50 can further be reduced.

Heat Treatment Conditions

The heat treatment temperature is preferably not less than 1000° C. andnot more than 2000° C. With a heat treatment temperature of not lessthan 1000° C., implanted ions easily gather in a planar form in ionimplantation layer 15 and easily form air bubbles. Thus, fine voids areeasily formed to spread two-dimensionally in ion implantation layer 15.Moreover, with a heat treatment temperature of not less than 1000° C.,bonds between carbon atoms forming the diamond crystal structure areeasily severed. Under these conditions, separation of diamond substrate50 is facilitated.

Further, with a heat treatment temperature of not less than 1000° C.,the heat treatment can be performed at a temperature higher than thegrowth temperature for diamond growth layer 20. Thus, the method ofmanufacturing diamond substrate 50 in the present embodiment accordingto which diamond growth layer 20 is grown and thereafter heat treatmentis performed to separate diamond substrate 50 can be carried out.

With a heat treatment temperature of not more than 2000° C.,transformation of diamond structure 30 to graphite or amorphous carbondue to the heat treatment can be prevented. Thus, diamond substrate 50excellent in crystal quality can be manufactured. The heat treatmenttemperature is more preferably not less than 1200° C. and not more than1500° C.

The heat treatment temperature is still more preferably not more than1400° C. Under this condition, not only when a Type IIa diamondsubstrate is used as single crystal diamond seed substrate 10, but alsowhen a Type Ib diamond substrate is used as single crystal diamond seedsubstrate 10, single crystal diamond substrate 50 can be separated.

Unless the heat treatment temperature is a critical temperature or more(the temperature at which bonds between carbon atoms forming the diamondcrystal structure can be severed), diamond substrate 50 is notseparated. When the heat treatment temperature is the criticaltemperature or more, the heat treatment temperature and the heattreatment time (the time for which the temperature of diamond structure30 is kept at the above-described heat treatment temperature) can beused to calculate the activation energy necessary for separating diamondsubstrate 50. Therefore, whether diamond substrate 50 can be separatedor not is determined depending on both the heat treatment temperatureand the heat treatment time. When the heat treatment temperature ishigh, diamond substrate 50 is separated even if the heat treatment timeis short. In contrast, when the heat treatment temperature is thecritical temperature or more but the heat treatment temperature is notsufficiently high, the heat treatment time has to be extended toseparate diamond substrate 50.

When the heat treatment time is less than five minutes, separation ofdiamond substrate 50 may not reliably occur due to a variation of theanneal furnace or heat-treated samples. On the contrary, when the heattreatment time is longer than 10 hours, the cost for separation ofdiamond substrate 50 is high, resulting in difficulty in mass productionof diamond substrate 50. The heat treatment time is therefore preferablynot less than 5 minutes and not more than 10 hours, and the heattreatment time is preferably set in consideration of the heat treatmenttemperature as described above.

The heat treatment is preferably performed in an inert gas atmosphere ora vacuum atmosphere at 1×10⁻² Pa or less. When the heat treatment isperformed in an inert gas atmosphere, transformation of diamondstructure 30 to graphite or amorphous carbon due to heat treatment canbe prevented. “Inert gas” herein may be noble gas such as helium, neon,or argon, may be nitrogen gas, or may be a mixture of noble gas andnitrogen gas. When the heat treatment is performed in a vacuum at 1×10⁻²Pa or less, transformation of diamond structure 30 to graphite oramorphous carbon due to the heat treatment can be prevented.

In the heat treatment atmosphere, the presence of even a small amount ofoxygen causes transformation of diamond to graphite or amorphous carbon.Therefore, oxygen has not been regarded as a material to be fed when theheat treatment is performed at 1000° C. or more. However, in the heattreatment step, an extra non-diamond layer in a region to be divided maynot be sufficiently removed to hinder the separation of the diamondsubstrate and cause breakage. In view of this, it has been found that atiny amount of oxygen gas or a tiny amount of gas containing oxygenatoms can be fed during the heat treatment to eliminate the factorhindering the separation and thereby smoothly separate the diamondsubstrate. If the amount of the oxygen gas or the gas containing oxygenatoms is excessively large, diamond is transformed to graphite oramorphous carbon. If this amount is excessively small, the non-diamondlayer such as graphite layer or amorphous carbon layer hindering theseparation cannot be removed. Therefore, the concentration of the oxygengas in the inert gas atmosphere is preferably not less than 10 ppm andnot more than 1000 ppm and more preferably not less than 20 ppm and notmore than 100 ppm. While the pressure in the inert gas atmosphere is notparticularly limited, this pressure is preferably at least a vacuumpressure and at most 4 GPa. Since high-pressure annealing requires ahigh cost, the pressure is preferably at least a vacuum pressure and atmost an atmospheric pressure for the sake of enabling reduction of thecost. While the vacuum pressure herein is not particularly limited, thevacuum pressure is preferably not less than 1×10⁻⁸ Pa and not more than1×10⁻² Pa. Moreover, the partial pressure of the oxygen gas in thevacuum atmosphere at not less than 1×10⁻⁸ Pa and not more than 1×10⁻² Pais preferably not less than 1×10⁻⁸ Pa and not more than 1×10⁻⁵ Pa, andmore preferably not less than 1×10⁻⁸ Pa and not more than 1×10⁻⁷ Pa.

[Structure of Diamond Substrate]

In the following, a description will be given of diamond substrate 50manufactured in accordance with the (above-described) method ofmanufacturing diamond substrate 50 in the present embodiment. Diamondsubstrate 50 manufactured by the above-described method has a highcrystal quality. As long as diamond substrate 50 in the presentembodiment has the structure and characteristics described below,diamond substrate 50 may be a diamond substrate manufactured inaccordance with a different method from the above-described method.

FIG. 3 shows a cross-sectional view of diamond substrate 50 and aschematic diagram of a concentration distribution of first atoms in arelevant part of diamond substrate 50. Diamond substrate 50 has a firstsurface (upper surface) 51 and a second surface (lower surface) 52located opposite to the first surface. Preferably, diamond substrate 50has a thickness T of not less than 100 μm and not more than 1500 μm, andhas a diameter R of not less than 50.8 mm (not less than two inches).Further, diamond substrate 50 of a single crystal has a diameter R ofpreferably not less than 101.6 mm (four inches in diameter), morepreferably not less than 152.4 mm (six inches in diameter). Then,diamond substrate 50 can be used as a substrate to be mounted with adevice, and the substrate to be mounted with a device can therefore bemanufactured in a short time and at a low cost. Preferably, diamondsubstrate 50 further has at least one of the following characteristics.

Concentration Distribution of First Atoms

First surface 51 is a surface formed by growth of diamond growth layer20. Second surface 52 is a surface formed by separation of diamondsubstrate 50. Therefore, concentration C_(H) of first atoms at secondsurface 52 is higher than the concentration of first atoms at firstsurface 51. As described above, actually ions of first atoms aredistributed in diamond substrate 50 as illustrated below. Thedistribution of first atoms as illustrated below is common to singlecrystal diamond substrate 50 and polycrystal diamond substrate 50.

Specifically, in diamond substrate 50, the concentration of first atomsdecreases from second surface 52 toward the inside of diamond substrate50 in the direction of thickness T of diamond substrate 50. Moreover,concentration C_(H) of first atoms at second surface 52 is not less than100 times as high as concentration C_(L) of first atoms in the inside ofdiamond substrate 50 (FIG. 2 ). For example, concentration C_(H) offirst atoms at second surface 52 of diamond substrate 50 is not lessthan 1×10²⁰ cm⁻³, and concentration C_(L) of first atoms in the insideof diamond substrate 50 is not more than 1×10¹⁸ cm⁻³.

“First atoms at second surface 52” herein refer to first atoms implantedinto second surface 52 by ion implantation. Therefore, “first atoms atsecond surface 52” do not include first atoms naturally adhering to thesecond surface (as described above) and do not include first atoms takeninto the second surface from the atmosphere with which the secondsurface is in contact. The first atoms taken into the second surfacefrom the atmosphere with which the second surface is in contact are onlypresent at a depth on the order of several nm from the second surface.

Flatness of Second Surface

When diamond substrate 50 is manufactured by the above-described method,namely by performing heat treatment on the ion implantation layer togenerate many fine air bubbles and cause diamond substrate 50 to beseparated by the gas dissociation pressure of the air bubbles, theformed air bubbles generate flat regions and a protrusion/depressionhaving a height of not less than 1 nm and not more than 50 nm is formedaround each of the flat regions, and the region of theprotrusion/depression is a region which has been forced to be separated.Thus, a mesh structure is formed. When the size of the air bubbles isless than 3 μm, the gas dissociation pressure required to separatediamond substrate 50 cannot be obtained. When the size of the airbubbles is more than 30 μm, the air bubble regions are partiallyseparated, however, due to a lower density of the air bubbles, theregion around the air bubbles is accordingly larger and diamondsubstrate 50 is difficult to separate. Therefore, preferably the flatregions each have a size of not less than 3 μm and not more than 30 μm.

When diamond substrate 50 is manufactured following the above-describedmethod, second ion implantation layer 15 b (FIG. 1 (C)) may be presentin second surface 52 of diamond substrate 50, and a graphite region oramorphous carbon region formed by the above-described heat treatment maybe present in second surface 52.

Since the ion implantation depth is preferably 3 μm or less, secondsurface 52 of diamond substrate 50 is considered as relatively flat. Inview of the above, it is considered that second surface 52 can bechemically polished to remove second ion implantation layer 15 b or thelike from second surface 52 to use diamond substrate 50 as a substrateto be mounted with a device. The method of chemically polishing secondsurface 52 may be a method to chemically polish second surface 52 withhot mixed acid. “Chemical polishing with hot mixed acid” herein refersto etching at 200° C. with nitric acid:sulfuric acid=1:3 (volume ratio).

Before the chemical polishing with hot mixed acid, a protrusion having aheight on the order of 100 nm or a depression having a depth on theorder of 100 nm is present in second surface 52. Theprotrusion/depression is considered as being generated due to second ionimplantation layer 15 b or a graphite region or amorphous carbon regionor the like formed by the heat treatment as described above.

However, when the chemical polishing with hot mixed acid is performed,second ion implantation layer 15 b and the graphite region and theamorphous carbon region or the like formed by the heat treatment areremoved, and accordingly second surface 52 has an arithmetic meanroughness Ra of 5 nm to 50 nm.

In addition, after the chemical polishing with hot mixed acid isperformed, a mesh structure is present in at least a part of secondsurface 52. The mesh structure is defined by a protrusion/depressionhaving a height of not less than 1 nm and not more than 50 nm andsurrounding each of flat regions. The flat regions at second surface 52each have a size L of not less than 3 μm and not more than 30 μm.Namely, by the chemical polishing with hot mixed acid, the non-diamondlayer formed by the heat treatment is removed to generate the meshstructure with the lessened protrusion/depression. A reason why such amesh structure is formed at second surface 52 may be as follows. Thismesh structure is present in at least a part of second surface 52 ofdiamond substrate 50 of either a single crystal or a polycrystal, afterthe chemical polishing.

In the regions generated by air bubbles which are formed by ionsgathered through the heat treatment, separation of diamond substrate 50spontaneously occurs. Therefore, after second surface 52 is chemicallypolished with hot mixed acid, flat regions are formed at second surface52.

Meanwhile, in the region where ions are difficult to gather even by theheat treatment, diamond substrate 50 is forced to be separated.Therefore, after second surface 52 is chemically polished with hot mixedacid, a protrusion/depression with a height of not less than 1 nm andnot more than 50 nm is formed. The protrusion/depression of secondsurface 52 is measured with an AFM (Atomic Force Microscope) or confocallaser microscope.

Even when such a mesh structure is formed in second surface 52 ofdiamond substrate 50, this diamond substrate 50 can be used as asubstrate to be mounted with a device. Since second surface 52 can bechemically polished to manufacture the substrate to be mounted with adevice, without physically polishing second surface 52, the substrate tobe mounted with a device can be manufactured at a low cost and in ashort time.

Further, preferably second surface 52 is physically polished. Secondsurface 52 can be physically polished to further lessen theprotrusion/depression. Thus, diamond substrate 50 with second surface 52physically polished can be used as a freestanding substrate.

Since first surface 51 is a surface formed by growth of diamond growthlayer 20, first surface 51 has a larger arithmetic mean roughness thansecond surface 52 before polished. It is therefore preferable tophysically polish first surface 51 by laser-beam machining, slicing, orpolishing or the like.

Light Emission Characteristics

FIG. 4 schematically shows a photoluminescence (PL) spectrum of singlecrystal diamond substrate 50 obtained by applying, to single crystaldiamond substrate 50, excitation light having a peak wavelength in awavelength range of not less than 315 nm and not more than 335 nm at atemperature in a temperature range of not less than 7 K and not morethan 83 K. As shown in FIG. 4 , the photoluminescence spectrum of singlecrystal diamond substrate 50 includes a first emission peak 101 and asecond emission peak 102.

First emission peak 101 has an emission peak wavelength in a wavelengthrange of not less than 450 nm and not more than 650 nm, and has a fullwidth at half maximum (FWHM shown in FIG. 4 ) of not less than 50 nm.Second emission peak 102 has an emission peak wavelength in a wavelengthrange of not less than 570 nm and not more than 580 nm, and has a fullwidth at half maximum of not more than 10 nm. A peak height I₁ of firstemission peak 101 is not less than 0.1 times ( 1/10 times), preferablynot less than 0.2 times (⅕ times), and more preferably not less than 0.5times (½ times) as high as a peak height I₂ of second emission peak 102.

“Peak height I₁ of first emission peak 101” herein refers to theemission intensity after background correction at a vertex P included infirst emission peak 101, and is calculated by the following formula.

(peakheightI₁offirstemissionpeak101) = (emissionintensityatvertexP) − (emissionintensityatthebottomoffirstemissionpeak101)

“Peak height I₂ of second emission peak 102” herein refers to theemission intensity after background correction at a vertex Q included insecond emission peak 102, and is calculated by the following formula.

(peakheightI₂ofsecondemissionpeak102) = (emissionintensityatvertexQ) − (emissionintensityoffirstemissionpeak101atthewavelengthatvertexQ)

It was found that the emission characteristics of single crystal diamondsubstrate 50 were changed by the heat treatment. Specifically, beforethe heat treatment, the photoluminescence spectrum of diamond growthlayer 20 had not only first emission peak 101 and second emission peak102 but also a third emission peak 103 (having a full width at halfmaximum of 10 nm or less) identified as having an emission peakwavelength on the lower-wavelength side relative to the wavelength atvertex P, in a wavelength range of not less than 450 nm and not morethan 650 nm. However, after the heat treatment, third emission peak 103was not identified (FIG. 4 ).

Moreover, before the heat treatment, peak height I₁ of first emissionpeak 101 was less than 0.1 times ( 1/10 times) as high as peak height 12of second emission peak 102. However, by the heat treatment, peak heightI₁ of first emission peak 101 was increased while peak height 12 ofsecond emission peak 102 was decreased. As a result of this, asdescribed above, peak height I₁ of first emission peak 101 was not lessthan 0.1 times ( 1/10 times), preferably not less than 0.2 times (⅕times), and more preferably not less than 0.5 times (½ times) as high aspeak height 12 of second emission peak 102. Thus, the emissioncharacteristics of the single crystal diamond substrate can be examinedto determine whether or not the single crystal diamond substrate hasbeen subjected to the heat treatment.

Absorption Coefficient

FIG. 5 schematically shows an absorption spectrum of single crystaldiamond substrate 50, with the wavelength plotted on the horizontal axisand the absorption coefficient plotted on the vertical axis. As shown inFIG. 5 , the absorption spectrum of single crystal diamond substrate 50includes a first absorption peak 111 having an absorption peakwavelength in a wavelength range of not less than 265 nm and not morethan 275 nm, and a second absorption peak 112 having an absorption peakwavelength in a wavelength range of not less than 370 nm and not morethan 390 nm. A peak height α₂ of second absorption peak 112 is lowerthan a peak height al of first absorption peak 111, and is preferablyless than 0.5 times (½ times) as high as peak height α₁.

The absorption spectrum shown in FIG. 5 is calculated in accordance withthe following method. A xenon lamp is used as a light source, the lightwith a wavelength range of not less than 200 nm and not more than 850 nmis split by a diffraction grating into its constituent colors, the lightsplit into its constituent colors is applied to single crystal diamondsubstrate 50, and the transmittance of single crystal diamond substrate50 is measured. A part of the light applied to single crystal diamondsubstrate 50 is transmitted through single crystal diamond substrate 50,another part thereof is absorbed by single crystal diamond substrate 50,and still another part thereof is reflected from the surface of singlecrystal diamond substrate 50 or may undergo multiple reflection from thesurface of single crystal diamond substrate 50. In view of the above,the absorption coefficient of single crystal diamond substrate 50 iscalculated.

“Peak height α₁ of first absorption peak 111” refers to the absorptioncoefficient after background correction at vertex R included in firstabsorption peak 111. Likewise, “peak height α₂ of second absorption peak112” herein refers to the absorption coefficient after backgroundcorrection at vertex S included in second absorption peak 112. For theabsorption spectrum of single crystal diamond substrate 50 shown in FIG.5 , the background correction is made in accordance with the followingmethod. In a formula expressing the absorption coefficient at each ofvertex R and vertex S, the terms proportional to the first power, thesecond power, and the third power to which the wavelength is raised arecancelled, and only the term which is not proportional to the wavelengthis extracted.

It was found that the absorption coefficient of single crystal diamondsubstrate 50 was changed by the heat treatment. Specifically, before theheat treatment, peak height α₂ of second absorption peak 112 was higherthan peak height α₁ of first absorption peak 111, and even not less than1.2 times as high as peak height α₁ of first absorption peak 111 in somecases. However, after the heat treatment, peak height α₁ of firstabsorption peak 111 was higher and peak height α₂ of second absorptionpeak 112 was lower. As a result of this, as described above, peak heightα₂ of second absorption peak 112 was lower than peak height α₁ of firstabsorption peak 111 and preferably less than 0.5 times (½ times) as highas peak height α₁ of first absorption peak 111. Thus, the absorptioncoefficient of the single crystal diamond substrate can be examined todetermine whether or not the single crystal diamond substrate has beensubjected to the heat treatment.

Diamond substrate 50 of a polycrystal is obtained by the vapor phasesynthesis method, and the average grain size of crystal grains in thepolycrystal of diamond substrate 50 is not less than 30 μm, preferablynot less than 60 μm, and more preferably not less than 90 μm. Suchpolycrystal diamond substrate 50 is easily separated from polycrystaldiamond structure 30 and has a high crystal quality. The average grainsize of crystal grains in the polycrystal of polycrystal diamondsubstrate 50 is measured with an SEM.

Resistivity

As described above, even when diamond structure 30 is electricallyconductive, the heat treatment as described above can be performed ondiamond structure 30 to separate diamond substrate 50. Therefore,diamond substrate 50 may include a layer having a resistivity of notless than 10⁻⁵ Ω·cm and not more than 10⁻² Ω·cm to exhibit electricalconductivity, not less than 10⁶ Ω·cm and not more than 10⁹ Ω·cm toexhibit electrical insulation, or a resistivity between them.Alternatively, diamond substrate 50 as a whole may have a resistivity ofnot less than 10⁻⁵ Ω·cm and not more than 10⁻² Ω·cm to exhibitelectrical conductivity, not less than 10⁶ Ω·cm and not more than 10⁹Ω·cm to exhibit electrical insulation, or a resistivity between them. Inany case, the heat treatment can be performed to separate diamondsubstrate 50.

[Use of Diamond Substrate]

Diamond substrate 50 can be used as a substrate to be mounted with adevice. For example, on first surface 51 or second surface 52 of diamondsubstrate 50, a diamond growth layer doped with an impurity or a diamondlayer terminated with hydrogen atoms is formed. After this, an electrodeand the like are formed. In this way, an electronic device(semiconductor device such as power semiconductor device orhigh-frequency semiconductor device, UV-emitting device, electronemission source, magnetic sensor, or biosensor, for example) can bemanufactured.

Diamond substrate 50 may be used to fabricate a diamond compositesubstrate 70 described later herein, and use this diamond compositesubstrate 70 as a substrate to be mounted with a device.

In the case where diamond substrate 50 is used as a substrate to bemounted with a device, the second surface of diamond substrate 50 may bephysically polished instead of being chemically polished. While the costfor chemical polishing is low, the chemically polished second surface issomewhat uneven, and therefore, the use of such a diamond substrate islimited to a device substrate on which vapor phase deposition isperformed, tools, electrodes, and the like. In contrast, while the costfor physical polishing is somewhat higher than the cost for chemicalpolishing, the resultant flatness of second surface 52 of diamondsubstrate 50 is higher. Therefore, this diamond substrate 50 isapplicable to uses requiring flatness. Preferably, the surface treatmentmethod for second surface 52 is selected depending on the use of diamondsubstrate 50.

[Structure of Diamond Composite Substrate]

FIG. 6 is a cross-sectional view of a diamond composite substrate 70 inthe present embodiment. Diamond composite substrate 70 includes adiamond substrate 50 and an epitaxial layer 60 formed on a secondsurface 52 of diamond substrate 50. The method of forming epitaxiallayer 60 and the material for epitaxial layer 60 are not particularlylimited. Since diamond substrate 50 can be manufactured at a low costand in a short time, diamond composite substrate 70 including diamondsubstrate 50 can also be manufactured at a low cost and in a short time.Diamond substrate 50 of diamond composite substrate 70 may be either asingle crystal or a polycrystal.

In the case where diamond composite substrate 70 is used as a substrateto be mounted with a device, the second surface of diamond substrate 50may be physically polished instead of chemically polished. The flatnessof second surface 52 of diamond substrate 50 is accordingly higher.Preferably the surface treatment method for second surface 52 isselected depending on the use of diamond composite substrate 70.

EXAMPLES

The present invention will be hereinafter described in further detailbased on Examples. The present invention, however, is not limited tothem.

Example 1

Initially, a diamond seed substrate was produced. Specifically, a TypeIb diamond substrate (0.3 mm in thickness) produced by thehigh-temperature high-pressure method was prepared. On the upper surfaceof the diamond substrate, a single crystal CVD diamond layer (0.2 mm inthickness) was formed by the CVD method. The upper surface of thediamond substrate was a surface having an off angle of 5 degrees or lessrelative to plane orientation (001). After this, diamond abrasive grainswere used to polish the surface of the formed single crystal CVD diamondlayer. In this way, a CVD diamond substrate (diamond seed substrate) wasobtained. In the obtained CVD diamond substrate, the Type Ib diamondsubstrate had a thickness of 0.3 mm and the single crystal CVD diamondlayer had a thickness of 0.2 mm Next, under the conditions indicatedbelow, ions were implanted into a main surface (4 mm×4 mm in size) ofthe single crystal CVD diamond layer of the CVD diamond substrate. Inthis way, an ion implantation layer was formed at a side of the mainsurface of the single crystal diamond layer of the CVD diamondsubstrate. The ion implantation depth was about 0.4 μm. The variation ofthe ion implantation depth was about 30 nm.

Ion Implantation Conditions

-   -   kind of ions: hydrogen ions (H⁺)    -   ion implantation energy: 90 keV    -   ion implantation dose: 7×10¹⁷ cm⁻²

Subsequently, on the main surface of the single crystal CVD diamondlayer of the CVD diamond substrate, a single crystal diamond growthlayer (0.3 mm in thickness) was epitaxially grown by the CVD method.After this, a laser beam was applied to a diamond crystal deposited onthe side surface of the ion implantation layer to remove the diamondcrystal. In this way, a diamond structure was obtained.

Subsequently, under the conditions indicated below, heat treatment wasperformed on the diamond structure, as a method for separation.Accordingly, the diamond structure was divided along the ionimplantation layer into a first structure and a single crystal diamondsubstrate. Namely, the single crystal diamond substrate was separated.

Conditions for Separation

-   -   treatment temperature: 1000° C.    -   treatment time: one hour    -   treatment atmosphere: vacuum atmosphere (1×10⁻³ Pa)    -   partial pressure of oxygen gas: 1×10⁻⁸ Pa

After this, nitric acid and sulfuric acid were mixed at a ratio of 1:3(volume ratio), and the resultant mixed acid was used to perform etchingat 200° C. for two hours on the separated surface, which was exposed bythis separation, of the single crystal diamond substrate. In this way,the single crystal diamond substrate with the separated surface havingan arithmetic mean roughness Ra of 5 nm or less was obtained.

Examples 2-7

Single crystal diamond substrates were manufactured in accordance with asimilar method to the above-described method in Example 1, except thatchanges were made as shown in Table 1 to the type of the diamond seedsubstrate, the size of the main surface or the nitrogen concentration atthe main surface, the kind of implanted ions, the ion implantationenergy, the ion implantation dose, or the method for separation orconditions for separation.

Comparative Example 1

The Type Ib diamond substrate used in Example 1 was prepared as adiamond seed substrate.

Next, under the conditions indicated below, ions were implanted, in twosteps, into the main surface (4 mm×4 mm in size) of the Type Ib diamondsubstrate. In this way, an ion implantation layer was formed. Theconcentration of ions implanted into the diamond seed substrate wassimulated, and this ion concentration was estimated at 1.4×10²⁰ cm⁻³ to2.2×10²¹ cm⁻³. While the ion implantation layer was electricallyconductive, the Type Ib diamond substrate except for the ionimplantation layer was electrically insulating. The ion implantationdepth was about 0.38 μm.

Ion Implantation Conditions

-   -   kind of ions: carbon ions (C⁺)    -   ion implantation energy: 350 keV, 280 keV    -   ion implantation dose: 1.2×10¹⁶ cm⁻², 4×10¹⁵ cm⁻² Subsequently,        on the main surface, into which ions were implanted, of the Type        Ib diamond substrate, a diamond growth layer (0.3 mm in        thickness) was grown by the CVD method. After this, a laser beam        was applied to a diamond crystal deposited on the side surface        of the ion implantation layer to remove the diamond crystal. In        this way, a diamond structure was obtained.

Subsequently, electrolytic etching was performed on the diamondstructure. Specifically, pure water (etching solution) was placed in anelectrolytic etching bath made of borosilicate glass. A pair of platinumelectrodes was placed in the etching solution with a distance of about 1cm between the electrodes, and the above-described diamond substrate wasdisposed between the platinum electrodes of the electrode pair in theetching solution. A voltage of 340 V was applied between the platinumelectrodes of the electrode pair, and the diamond substrate was lefttherein until the ion implantation layer was electrolytically etchedaway completely.

Comparative Example 2

A single crystal diamond substrate was manufactured in accordance withthe above-described method in Comparative Example 1, except that a TypeIb diamond substrate with a diameter of the main surface of two inches(50.8 mm) was used as a diamond seed substrate.

[Results and Analysis]

The results are shown in Table 1. In Table 1, “Type Ib+CVD layer”represents a single crystal CVD diamond seed substrate.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 ion diamondtype single single single single single implantation seed crystalcrystal crystal crystal crystal conditions substrate Type Ib + Type Ib +Type Ib + Type IIa Type Ib + CVD layer CVD layer CVD layer CVD layersize 4 mm × 2 inches 4 mm × 4 mm × 4 mm × of main 4 mm (50.8 mm) in 4 mm4 mm 4 mm surface diameter nitrogen 5 ppm or 5 ppm or 5 ppm or 0.1 ppmor 5 ppm or concentration less less less less less at main surface kindof ions hydrogen hydrogen hydrogen hydrogen hydrogen ion implantationenergy 90 keV 90 keV 90 keV 50 keV 200 keV ion implantation dose 7 ×10¹⁷ 7 × 10¹⁷ 1 × 10¹⁷ 7 × 10¹⁶ 7 × 10¹⁷ (cm⁻²) method and method heatheat heat heat heat conditions treatment treatment treatment treatmenttreatment for separation treatment temperature (° C.) 1000   1200  1400   1600   2000   treatment time (hours) 1 1 1 1 1 treatmentatmosphere vacuum vacuum inert gas inert gas inert gas atmosphereatmosphere (nitrogen gas) (nitrogen gas) (nitrogen gas) (1 × 10⁻³ Pa) (1× 10⁻³ Pa) atmosphere atmosphere atmosphere concentration or partial 1 ×10⁻⁸ Pa 1 × 10⁻⁶ Pa 10 10 100 pressure of oxygen gas ppm ppm ppm peakintensity ratio in PL spectrum 0.125 0.25 1 2 5 (1st emission peak/2ndemission peak) absorption peak intensity ratio 0.8  0.7    0.4   0.2  0.1 (2nd absorption peak/1st absorption peak) separated or notseparated separated separated separated separated ComparativeComparative Example 6 Example 7 Example 1 Example 2 ion diamond typesingle single single single implantation seed crystal crystal crystalcrystal conditions substrate Type Ib + Type Ib Type Ib Type Ib CVD layersize 4 mm × 4 mm × 4 mm × 2 inches of main 4 mm 4 mm 4 mm (50.8 mm) insurface diameter nitrogen 5 ppm or 3-100 3-100 3-100 concentration lessppm ppm ppm at main surface kind of ions helium hydrogen carbon carbonion implantation energy 90 keV 90 keV 350 keV 350 keV 280 keV 280 keVion implantation dose 4 × 10¹⁷ 7 × 10¹⁷ 1.2 × 10¹⁶ 1.2 × 10¹⁶ (cm⁻²)   4× 10¹⁵   4 × 10¹⁵ method and method heat heat electrolytic electrolyticconditions treatment treatment etching etching for separation treatmenttemperature (° C.) 1200   1400   — — treatment time (hours) 1 1 13   100     treatment atmosphere inert gas inert gas — — (nitrogen gas)(nitrogen gas) atmosphere atmosphere concentration or partial 100 1000 —— pressure of oxygen gas ppm ppm peak intensity ratio in PL spectrum0.25 1 0.083 0.083 (1st emission peak/2nd emission peak) absorption peakintensity ratio 0.7    0.4 2   2   (2nd absorption peak/1st absorptionpeak) separated or not separated separated separated not separated (13hours) (100 hours)

As shown in Table 1, regarding Examples 1 to 7, the single crystaldiamond substrate could be separated by the heat treatment for one hour.In contrast, regarding Comparative Example 1, the single crystal diamondsubstrate could be separated by electrolytic etching for 13 hours and,regarding Comparative Example 2, the single crystal diamond substratecould not be separated even by the electrolytic etching for 100 hours.Thus, regarding the Examples, single crystal diamond substrates having alarge area ranging from 4 mm×4 mm to a diameter of two inches could beobtained in a short time of one hour by a low-cost method, namely heattreatment.

In Example 1, a single crystal diamond substrate of 4 mm×4 mm wasmanufactured. In Example 2, a single crystal diamond substrate with adiameter of two inches (50.8 mm) was manufactured. However, Example 1and Example 2 were identical to each other in terms of the time takenfor separating the single crystal diamond substrate. It was accordinglyfound, regarding the method of manufacturing a single crystal diamondsubstrate in the Examples, the time taken for separating the singlecrystal diamond substrate did not depend on the size of the singlecrystal diamond substrate.

In Example 1, the ion implantation dose was 7×10⁻⁷ cm⁻², the heattreatment temperature was 1000° C., the heat treatment atmosphere was avacuum atmosphere at 1×10⁻³ Pa, and the partial pressure of oxygen gasfor the heat treatment was 1×10⁻⁸ Pa. In Example 3, the ion implantationdose was 1×10¹⁷ cm⁻², the heat treatment temperature was 1400° C., theheat treatment atmosphere was a nitrogen gas atmosphere containingnitrogen gas as an inert gas at 100 kPa (atmospheric pressure), and theconcentration of oxygen gas for the heat treatment was 10 ppm. However,Example 1 and Example 3 were identical to each other in terms of thetime taken for separating the single crystal diamond substrate. It wasaccordingly found, regarding the method of manufacturing a singlecrystal diamond substrate in the Examples, the single crystal diamondsubstrate could be separated by optimizing the treatment temperature,the treatment atmosphere, or the concentration or partial pressure ofoxygen gas for the heat treatment, depending on the ion implantationdose.

In Example 1, the diamond seed substrate was a single crystal CVDdiamond substrate (with a nitrogen concentration at the main surface of5 ppm or less), the ion implantation dose was 7×10¹⁷ cm⁻², the heattreatment temperature was 1000° C., the heat treatment atmosphere was avacuum atmosphere at 1×10⁻³ Pa, and the partial pressure of oxygen gasfor the heat treatment was 1×10⁻⁸ Pa. In Example 4, the diamond seedsubstrate was a single crystal Type IIa diamond substrate (with anitrogen concentration at the main surface of 0.1 ppm or less), the ionimplantation dose was 7×10¹⁶ cm⁻², the heat treatment temperature was1600° C., the heat treatment atmosphere was a nitrogen gas atmospherecontaining nitrogen gas as an inert gas at 100 kPa (atmosphericpressure), and the concentration of oxygen gas for the heat treatmentwas 10 ppm. However, Example 1 and Example 4 were identical to eachother in terms of the time taken for separating the single crystaldiamond substrate. It was accordingly found, regarding the method ofmanufacturing a single crystal diamond substrate in the Examples, thesingle crystal diamond substrate could be separated by optimizing thetreatment temperature, the treatment atmosphere, or the concentration orpartial pressure of oxygen gas for the heat treatment, depending on thenitrogen concentration at the main surface of the diamond seedsubstrate.

In Example 1, the ion implantation energy was 90 keV, the heat treatmenttemperature was 1000° C., the heat treatment atmosphere was a vacuumatmosphere at 1×10⁻³ Pa, and the partial pressure of oxygen gas for theheat treatment was 1×10⁻⁸ Pa. In Example 5, the ion implantation energywas 200 keV, the heat treatment temperature was 2000° C., the heattreatment atmosphere was a nitrogen gas atmosphere containing nitrogengas as an inert gas at 100 kPa (atmospheric pressure), and theconcentration of oxygen gas for the heat treatment was 100 ppm. However,Example 1 and Example 5 were identical to each other in terms of thetime taken for separating the single crystal diamond substrate. It wasaccordingly found, regarding the method of manufacturing a singlecrystal diamond substrate in the Examples, the single crystal diamondsubstrate could be separated by optimizing the treatment temperature,the treatment atmosphere, or the concentration or partial pressure ofoxygen gas for the heat treatment, depending on the ion implantationenergy.

In Example 1, ions of hydrogen atoms were implanted. In Example 6, ionsof helium atoms were implanted. However, Example 1 and Example 6 wereidentical to each other in terms of the time taken for separating thesingle crystal diamond substrate. It was accordingly found, regardingthe method of manufacturing a single crystal diamond substrate in theExamples, the single crystal diamond substrate could also be separatedby using ions of helium atoms instead of ions of hydrogen atoms.

In Example 1, the diamond seed substrate was a single crystal CVDdiamond substrate (with a nitrogen concentration at the main surface of5 ppm or less), the heat treatment temperature was 1000° C., the heattreatment atmosphere was a vacuum atmosphere at 1×10⁻³ Pa, and thepartial pressure of oxygen gas for the heat treatment was 1×10⁻⁸ Pa. InExample 7, the diamond seed substrate was a single crystal Type Ibdiamond substrate (with a nitrogen concentration at the main surface of3 to 100 ppm), the heat treatment temperature was 1400° C., the heattreatment atmosphere was a nitrogen gas atmosphere containing nitrogengas as an inert gas at 100 kPa (atmospheric pressure), and theconcentration of oxygen gas for the heat treatment was 1000 ppm.However, Example 1 and Example 7 were identical to each other in termsof the time taken for separating the single crystal diamond substrate.It was accordingly found, regarding the method of manufacturing a singlecrystal diamond substrate in the Examples, the single crystal diamondsubstrate could be separated by optimizing the treatment temperature,the treatment atmosphere, or the concentration or partial pressure ofoxygen gas for the heat treatment, even when a single crystal Type Ibdiamond substrate was used as a diamond seed substrate.

In contrast, in Comparative Example 1, although the single crystaldiamond substrate was separated, the time taken for the separation was13 hours. In Comparative Example 2, the single crystal diamond substratewas not separated even after 100 hours. Based on these results, it wasfound that the time taken for the separation was considerably longerwhen the single crystal diamond substrate was separated by electrolyticetching, as compared with Examples 1 to 7, and the time taken for theseparation was considerably longer when the size of the main surface ofthe single crystal diamond substrate was larger.

Example 8

Initially, a diamond seed substrate was produced. Specifically, as abase substrate, a polycrystal diamond substrate (152.4 mm in diameter(six inches in diameter), 0.3 mm in thickness, and 30 μm in averagegrain size of crystal grains in the polycrystal at the main surface)produced by the hot filament CVD method was prepared. On the uppersurface of the polycrystal diamond substrate, a polycrystal CVD diamondlayer (0.2 mm in thickness) was formed by the microwave plasma CVDmethod. After this, diamond abrasives were used to polish the formedpolycrystal CVD diamond layer from the surface to a depth of 0.1 mm. Inthis way, a polycrystal CVD diamond substrate (diamond seed substrate)was obtained. The obtained polycrystal CVD diamond substrate had athickness of 0.4 mm. The average grain size of crystal grains in thepolycrystal at the main surface of the polycrystal diamond CVDlayer-side was measured with an SEM, and the average grain size was 30μm or more.

Next, under the conditions indicated below, ions were implanted into amain surface (152.4 mm in diameter (six inches in diameter)) of thepolycrystal CVD diamond layer of the polycrystal CVD diamond substrate.In this way, an ion implantation layer was formed at a side of the mainsurface of the polycrystal CVD diamond layer of the CVD diamondsubstrate. The ion implantation depth was about 0.4 μm. The variation ofthe ion implantation depth was about 30 nm.

Ion Implantation Conditions

-   -   kind of ions: hydrogen ions (H⁺)    -   ion implantation energy: 90 keV    -   ion implantation dose: 7×10⁻⁷ cm⁻²

Subsequently, on the main surface of the polycrystal diamond layer ofthe polycrystal CVD diamond substrate, a diamond growth layer (0.3 mm inthickness) was epitaxially grown by the CVD method. After this, a laserbeam was applied to a diamond crystal deposited on the side surface ofthe ion implantation layer to remove the diamond crystal. In this way, adiamond structure was obtained.

Subsequently, under the conditions indicated below, heat treatment wasperformed on the diamond structure, as a method for separation.Accordingly, the diamond structure was divided along the ionimplantation layer into a first structure and a polycrystal diamondsubstrate. Namely, the polycrystal diamond substrate was separated.

Conditions for Separation

-   -   treatment temperature: 1000° C.    -   treatment time: one hour    -   treatment atmosphere: vacuum atmosphere (1×10⁻² Pa)    -   partial pressure of oxygen gas: 1×10⁻⁵ Pa

After this, nitric acid and sulfuric acid were mixed at a ratio of 1:3(volume ratio), and the resultant mixed acid was used to perform etchingat 200° C. for two hours on the separated surface, which was exposed bythis separation, of the polycrystal diamond substrate. In this way, thepolycrystal diamond substrate with the separated surface having anarithmetic mean roughness Ra of 5 nm or less was obtained.

Examples 9, 10

Polycrystal diamond substrates were manufactured in accordance with asimilar method to the above-described method in Example 1, except thatchanges were made as shown in Table 2 to the size of the main surface ofthe diamond seed substrate, the average grain size of crystal grains inthe polycrystal of the main surface, or the nitrogen concentration atthe main surface, the kind of implanted ions, the ion implantationenergy, the ion implantation dose, or the method for separation orconditions for separation.

Comparative Example 3

The polycrystal CVD diamond substrate used in Example 8 was prepared asa diamond seed substrate.

Next, under the conditions indicated below, ions were implanted, in twosteps, into the main surface (152.4 mm in diameter (six inches indiameter)) of the polycrystal diamond seed substrate. In this way, anion implantation layer was formed. The concentration of ions implantedinto the diamond seed substrate was simulated, and this ionconcentration was estimated at 1.4×10²⁰ cm⁻³ to 2.2×10²¹ cm⁻³. While theion implantation layer was electrically conductive, the polycrystaldiamond substrate except for the ion implantation layer was electricallyinsulating. The ion implantation depth was about 0.38 μm.

Ion Implantation Conditions

-   -   kind of ions: carbon ions (C⁺)    -   ion implantation energy: 350 keV, 280 keV    -   ion implantation dose: 1.2×10¹⁶ cm⁻², 4×10¹⁵ cm⁻²

Subsequently, on the main surface, into which ions were implanted, ofthe polycrystal CVD diamond substrate, a polycrystalline diamond growthlayer (0.3 mm in thickness) was grown by the CVD method. After this, alaser beam was applied to a diamond crystal deposited on the sidesurface of the ion implantation layer to remove the diamond crystal. Inthis way, a diamond structure was obtained.

Subsequently, electrolytic etching was performed on the diamondstructure. Specifically, pure water (etching solution) was placed in anelectrolytic etching bath made of borosilicate glass. A pair of platinumelectrodes was placed in the etching solution with a distance of about 1cm between the electrodes, and the above-described diamond substrate wasdisposed between the platinum electrodes of the electrode pair in theetching solution. A voltage of 340 V was applied between the platinumelectrodes of the electrode pair, and the diamond substrate was lefttherein until the ion implantation layer was electrolytically etchedaway completely.

[Results and Analysis]

The results are shown in Table 2. In Table 2, “base+CVD layer”represents a polycrystal CVD diamond seed substrate.

TABLE 2 Comparative Example 8 Example 9 Example 10 Example 3 ion diamondtype polycrystal polycrystal polycrystal polycrystal implantation seedbase + base + base + base + conditions substrate CVD layer CVD layer CVDlayer CVD layer size of main 6 inches 8 inches 8 inches 6 inches surface(152.4 mm) in (203.2 mm) in (203.2 mm) in (152.4 mm) in diameterdiameter diameter diameter average crystal 30 60 90 30 grain size μm μmμm μm nitrogen 5 ppm or 5 ppm or 5 ppm or 5 ppm or concentration at lessless less less main surface kind of ions hydrogen hydrogen hydrogencarbon ion implantation energy 90 90 90 350 keV keV keV keV 280 keV ionimplantation dose (cm⁻²) 7 × 10¹⁷ 7 × 10¹⁷ 1 × 10¹⁷ 1.2 × 10¹⁶   4 ×10¹⁵ method and method for separation heat treatment heat treatment heattreatment electrolytic conditions for etching separation treatmenttemperature (° C.) 1000 1200 1400 — treatment time (hours)   1   1   1100 treatment atmosphere vacuum vacuum inert gas — atmosphere atmosphere(nitrogen gas) (1 × 10⁻² Pa) (1 × 10⁻² Pa) atmosphere concentration orpartial pressure 1 × 10⁻⁵ Pa 1 × 10⁻⁴ Pa 10 — of oxygen gas ppm peakintensity ratio in PL spectrum — — —  5 (1st emission peak/2nd emissionpeak) absorption peak intensity ratio — — — — (2nd absorption peak/1stabsorption peak) separated or not separated separated separated notseparated (100 hours)

As shown in Table 2, regarding Examples 8 to 10, the polycrystal diamondsubstrate could be separated by the heat treatment for one hour. Incontrast, regarding Comparative Example 3, the polycrystal diamondsubstrate could not be separated even by electrolytic etching for 100hours. Thus, regarding the Examples, polycrystal diamond substrateshaving a large area ranging from six inches to eight inches in diametercould be obtained in a short time of one hour by a low-cost method,namely heat treatment.

In Example 8, a polycrystal diamond substrate having a diameter of sixinches (152.4 mm) was manufactured. In Example 9, a polycrystal diamondsubstrate having a diameter of eight inches (203.2 mm) was manufactured.However, Example 8 and Example 9 were identical to each other in termsof the time taken for separating the polycrystal diamond substrate. Itwas accordingly found, regarding the method of manufacturing apolycrystal diamond substrate in the Examples, the time taken forseparating the polycrystal diamond substrate did not depend on the sizeof the polycrystal diamond substrate.

In Example 8, the ion implantation dose was 7×10¹⁷ cm⁻², the heattreatment temperature was 1000° C., the heat treatment atmosphere was avacuum atmosphere at 1×10⁻² Pa, and the partial pressure of oxygen gasfor the heat treatment was 1×10⁻⁵ Pa. In Example 10, the ionimplantation dose was 1×10¹⁷ cm⁻², the heat treatment temperature was1400° C., the heat treatment atmosphere was a nitrogen gas atmospherecontaining nitrogen gas as an inert gas at 100 kPa (atmosphericpressure), and the concentration of oxygen gas for the heat treatmentwas 10 ppm. However, Example 8 and Example 10 were identical to eachother in terms of the time taken for separating the polycrystal diamondsubstrate. It was accordingly found, regarding the method ofmanufacturing a polycrystal diamond substrate in the Examples, thepolycrystal diamond substrate could be separated by optimizing thetreatment temperature, the treatment atmosphere, or the concentration orpartial pressure of oxygen gas for the heat treatment, depending on theion implantation dose.

In contrast, in Comparative Example 3, the polycrystal diamond substratewas not separated even after 100 hours. It was found that the time takenfor the separation was considerably longer when the polycrystal diamondsubstrate was separated by electrolytic etching, as compared withExamples 8 to 10, and the time taken for the separation was considerablylonger when the size of the main surface of the polycrystal diamondsubstrate was larger.

It should be construed that the embodiments and examples disclosedherein are given by way of illustration in all respects, not by way oflimitation. It is intended that the scope of the present invention isdefined by claims, not by the embodiments and examples described above,and encompasses all modifications and variations equivalent in meaningand scope to the claims.

REFERENCE SIGNS LIST

10 diamond seed substrate; 11 main surface; 15 ion implantation layer;15 a first ion implantation layer; 15 b second ion implantation layer;20 diamond growth layer; 30 diamond structure; 40 first structure; 50diamond substrate; 51 first surface; 52 second surface; 60 epitaxiallayer; 70 diamond composite substrate; 101 first emission peak; 102second emission peak; 103 third emission peak; 111 first absorptionpeak; 112 second absorption peak

The invention claimed is:
 1. A diamond substrate of a polycrystal, thediamond substrate being obtained by a vapor phase synthesis method, andcrystal grains in the polycrystal of the diamond substrate having anaverage grain size of not less than 30 μm, wherein the diamond substrateincludes: a first surface; and a second surface located opposite to thefirst surface, a concentration of first atoms at the second surface ishigher than a concentration of the first atoms at the first surface, thefirst atoms being different from atoms forming a diamond crystalstructure, the concentration of the first atoms decreases from thesecond surface toward an inside of the diamond substrate, in a thicknessdirection of the diamond substrate, and the concentration of the firstatoms at the second surface is not less than 100 times as high as theconcentration of the first atoms in the inside of the diamond substrate.2. The diamond substrate according to claim 1, wherein in at least apart of the second surface, a mesh structure defined by a protrusion ordepression with a height of not less than 1 nm and not more than 50 nmsurrounding flat regions is formed, and the flat regions of the secondsurface each have a size of not less than 3 μm and not more than 30 μm.